The technology for settling of solids suspended in liquids has been extensively developed over a long time period. A description of a review of an 80 year history of the development of liquid clarifiers used in conjunction with water and sewage works can be found in an article entitled "The Sludge Blanket Clarifier" published in Water and Sewage Works, Vol. 97, No. 4 in April of 1950. A large variety of liquid clarifiers are described in this article for use in removing suspended solids. A more recent description of techniques and devices related to the separation of solids from a liquid can be found in Chemical Engineering in a desk book publication issue thereof on Feb. 15, 1971 in an article entitled "Thickening and Clarification", authored by Messrs. Dahlstrome and Cornell. Various solid settling techniques are discussed such as a thickener involving an underflow pumping arrangement, the use of special chemicals to enhance the formation of flocculents as well as improve thickener operations and several specific clarifier designs.
In the analysis of solid-liquid separation techniques, the solid suspensions are divided generally into two basic categories, a non-flocculent particulate and a flocculent suspension formed of flocs and primary particles. The flocculent suspension has the ability to agglomerate into flocs also known as agglomerates, while the non-flocculent particulate is not able to do so. Some suspensions are naturally flocculent and create the flocs or agglomerates without any additives. Other suspensions require some chemical aid in order to coagulate and flocculate into flocs. These aids may be inorganic salts such as alum, ferric chloride or an organic material known as polyelectrolytes.
When gravity settling is used for the solid-liquid separation of the non-flocculent particulate, the sedimentation tank performance depends upon the hydraulic loading of the tank. Non-flocculent particles larger than a certain critical diameter settle completely for a given hydraulic load. Smaller non-flocculent particulates are removed only partially, depending upon the ratio of the settling velocity and the hydraulic loading.
It is well known and accepted that the sedimentation of flocculent matter depends upon a flocculation process. The flocculation process makes the agglomeration of small particles and small flocs into larger flocs possible and thus enables solids to settle under given conditions. In order to optimize the settling out of flocculent suspensions above a particular size, the traditional technique involves control over the hydraulic loading of the tank and the theoretical detention time of the liquid as it passes through the tank. Hydraulic loading specifications are defined so that flocs above a certain size will settle out and these specifications then determine the area of the tank. The detention time is set to allow small primary particles and fine flocs to reflocculate into larger settlable flocs. Such reflocculation is essential for efficient separation of flocculent suspensions.
The state of a flocculent suspension is dependent upon two processes: flocculation and break-up of flocs. These two processes act simultaneously. When the turbulence in the suspension is high, the rate of break-up dominates over the rate of flocculation. The resulting flocculent suspension is in the form of fine flocs and primary particles. However, when the turbulence (which can be expressed as an average velocity gradient, G) is low, the flocculation process dominates and larger settleable flocs are created. Both the rate of flocculation and break-up are directly dependent upon the concentration of suspended solids and the average velocity. gradient. The net rate of decrease in the amount of primary particles can be expressed by the relationship EQU (dn.sub.1 /dt) =-K.sub.a .multidot.Y.multidot.n.sub.1 .multidot.G+k.sub.b .multidot.Y.multidot.Gm
where
K.sub.a =aggregation rate coefficient PA1 Y=suspended solids concentration PA1 n.sub.1 =primary particle concentration PA1 t=time PA1 G=velocity gradient PA1 K.sub.b =floc break-up rate coefficient PA1 m=floc break-up rate exponent.
From the above relationship one may appreciate that in order to optimize the performance of a sedimentation tank, such as may be used as a clarifier or thickener, it is essential to control the flocculation of the flocculent suspension by controlling the velocity gradient, G. This may be understood in describing the conditions prevailing in a sedimentation tank.
For example, an activated mixed sludge liquor, which is brought into a final clarifier from an aeration basin, includes both non-flocculent particulate and a flocculent suspension formed of small flocs and primary particles produced from the highly turbulent conditions in the basin which G could be well over 150 sec.sup.-1 (meters/sec -meter). Such turbulence is necessary for efficient oxygen transfer but the small flocs and primary particles need to reflocculate into larger agglomerates to settle them in the final clarifier tank. During the passage of the liquor from the aeration basin to the clarifier sedimentation tank, some reflocculation of small primary particles and flocs may occur. Such reflocculation is enhanced if the velocity gradient of the liquor in its travel through the conduits,, from the aeration basin and the feed well leading into the clarifier drops to a level where the rate of break-up of flocs is exceeded by the rate of flocculation. If flocculation is permitted to increase, the liquor arrives in the clarifier with larger flocs and less primary particles to thus enhance the removal efficiency of the clarifier.
The precise range of values of G in which the flocculation process dominates over the break-up of flocs may vary depending primarily upon material. One may judge from an article entitled "Physical Conditioning of Activated Sludge Floc" by D. S. Parker, W. J. Kaufman and D. Jenkins, published in The Water Pollution Control Federation Journal of September 1971 at pages 1817-1833, that at values of G generally below 30-40 sec.sup.-1 the flocculation process is dominant.
On the other hand, if the mixed liquor along its travel path from the aeration basin to the clarifier sedimentation tank encounters an increase in the velocity gradient, a break-up of flocs tends to occur resulting in a lower removal efficiency of the clarifier. Such an increase in the velocity gradient usually occurs at the entrance to the sedimentation tank, causing high velocity gradients in the region inside and around the entrance of the tank. It is thus desirable that the kinetic energy of the incoming liquor is dissipated without a break-up of flocs.
A variety of feeding arrangements have been proposed to dissipate kinetic energy of incoming liquor. For example, with reference to an early 1930 U.S. Pat. No. 1,754,119 to S. Pink, an apparatus for separating liquids of different densities is described. The incoming liquor is applied through cones fed from an inlet conduit into a settling tank. The cone angles are described to serve the function of reducing the incoming velocity of the liquid before it actually gets into the separating tank. The speed reduction described may be from 150 feet per minute down to 2 feet per minute at the place where the liquor enters the sedimentation tank. The cones are described as having a circular cross-section though rectangular designs may be employed with the flare angle to be of any suitable magnitude.
In the U.S. Pat. No. 2,098,467 to Sayers, et al, a diffuser is associated with a settling tank and interposed between an inlet conduit and a tank. The diffuser reduces the velocity of the incoming liquor to improve distribution. The diffuser is employed in conjunction with a vortex generating apparatus, which not only creates a vortex motion of the liquor, but also causes the liquor to spread and diffuse laterally across the tank from the top to avoid direct flow of liquor from the fluid's point of entry of the tank outlets. The presence of the vortex elements introduces shear forces on the incoming liquor, thus tending to break up flocs.
In the U.S. Pat. No. 2,343,836 to Webber, a nozzle is described to introduce liquid into a clarifier tank in such manner that the velocity of the liquid flow relative to a set of paddles is of the order of 90 feet per minute. The velocity of the liquid introduced into the settling tank tends to create highly turbulent conditions, thus increasing the break-up of flocs rather than their formation.
In the U.S. Pat. No. 2,947,380 to Fullaway, an introductory passageway is described whose cross-sectional size constantly increases so that the influent liquor encounters a constantly and gradually decreasing velocity.
In a more recent U.S. Pat. No. 3,456,798, a diffuser is described for introducing influent liquor into a clarifier tank. The diffusers described are best illustrated with reference to the '798 patent's FIGS. 7 and 8 wherein slot openings of gradually changing dimensions are shown. These slots are asserted to establish an essentially constant entrance velocity of the liquid into the reservoir. The liquid is introduced into the reservoir at a velocity which may be in the range from 6 to 240 feet per minute, but preferably is in the range from about 30 to 60 feet per minute.
With these diffusers and devices as described in the aforementioned patents, a velocity reduction of the incoming liquor can be achieved but not necessarily without shear conditions tending to break up flocculations. For example, the mere use of a diffuser to introduce liquor into a sedimentation tank does not assure that the velocity gradient (G) of the liquor is in the range where the flocculation process is dominant. Hence, the incoming stream of liquor may extend well into the sedimentation tank where the dissipation of the stream's kinetic energy is accompanied by turbulence and high shear forces tending to break flocs apart.
As noted with the above relationship for the reduction of primary particles, the flocculation process depends upon the concentration of suspended solids. When an incoming liquor is introduced in a sedimentation tank, a substantial dilution takes place resulting in a substantial reduction of the concentration of suspended solids. Such dilution impedes the flocculation process and is, therefore, preferably avoided.
Submerged inlet techniques have been described in the art to introduce the incoming liquor into a sedimentation tank in such manner that the effect of the above dilution upon the flocculation process is reduced. One such technique may be as described with respect to so-called up-flow clarifiers where the influent liquor is introduced at the bottom of the tank below a blanket of flocs through which the liquor must travel. Although the presence of the blanket enhances the formation of flocs, such up-flow clarifiers are sensitive to flow fluctuation and have limited applications.